Fatigue-limited metal components of gas turbines or jet engines, or other machine components subject to metal fatigue or failure, are carefully managed in order to avoid failure during operation. The failure, for example, of a critical component of a jet engine during operation may result in the loss of life or other catastrophic consequences. Therefore, the industry researches, designs, and produces components to provide greater strength and durability to avoid such situations.
Single crystal (SX) materials and directionally solidified (DS) materials have considerable advantages over materials cast using conventional methods. For example, in conventionally cast materials, there are large numbers of grain boundaries. These grain boundaries often present weakness points in the plane normal to the loading direction along which premature damage can occur. In addition, the low modulus of the grain in the loading direction helps reduce the stress applied to the material. These grain boundaries in the plane normal to the loading direction are essentially eliminated in SX and DS materials. Rather, the SX and DS materials have a more uniform structure and grain orientation. This uniform structure imparts enhanced directional strength characteristics and enhanced thermal fatigue resistance. These characteristics make DS and SX materials ideal for use in applications where strength and heat resistance are paramount. For example, SX and DS materials are used in the formation of turbine blades.
In addition to their favorable strength and heat resistance properties, materials incorporating metals or other substances which have directionally solidified grains (DS) or are composed of a single crystal (SX) are used in applications where the combination of exceptional strength and light weight are important. These materials are known as anisotropic materials, meaning they have directionally associated properties. When designing components or parts that are to be composed of DS or SX materials, the orientation of the grain of the crystal is aligned to provide the highest amount of strength to provide exceptionally strong and fatigue resistant materials without having to increase their weight to achieve these properties. Ideally, these DS and SX materials have an extended service life and are less prone to catastrophic failure. For example, DS and SX material are ideal for use in applications such as turbine blades in jet engines. However, if during production, the grain of the crystal is incorrectly oriented with respect to the direction of high stress loading the part experiences in service, the optimum strength characteristics may not be realized.
Single crystal materials are formed of a crystalline solid wherein the crystalline lattice of the material is continuous and unbroken to the edges of the material. Therefore, in SX material, there would not be grain boundaries within the material. Directionally solidified grain materials have a similar crystalline composition to single crystal materials, with the exception that there may be one or more grains, and therefore sets of grain boundaries parallel to the loading direction, within the material.
Forming SX and DS materials presents a number of production problems. An assortment of variables can affect the microstructure of solids. For example, the presence of impurities can affect the formation of the of the SX or DS material. Further, crystallographic defects and dislocations can occur during formation. For these reasons, single crystals of a significant size are rare in nature and are difficult to engineer even under strictly controlled conditions laboratory or industrial conditions. Grain boundaries can have an effect of the physical properties of materials relative to single crystal materials, but materials having directionally solidified grains can still offer significant strength improvements over materials not having directionally solidified grains.
Both SX and DS material have a crystalline structure which has a specific directional orientation that provides the optimum strength characteristics. It is this specific crystalline orientation that imparts the exceptional strength characteristic to these materials. Hence, it is in the interest of a manufacturer to control the formation of the SX or DS material to eliminate impurities and to control, or at least be aware of, the directionality of the crystalline or grain orientation in the material. Decisions to accept or reject the component are based on the orientation of the SX or DS crystal or grains.
During the formation of SX and DS materials, it is particularly important to avoid the occurrence of mal oriented grains (MOGs). The presence of MOGs in SX and DS material can have a significant impact on the directionally enhanced strength characteristics of the material. Once a SX or DS material has been formed, any imperfections or MOGs are an inherent part of the material and have an impact upon the strength characteristics of the material. Undetected MOGs can be a contributing factor in reducing the useful life of components or parts composed of SX and DX materials such as turbine blades. Additionally, it is vital that the grain orientation of the material correctly coincide with the appropriate direction of the desired enhanced strength of the part in order to ensure the desired properties of the SX and DS material in the blade.
Determining the grain orientation of an SX or DX material where the single crystal itself is exposed can be simply determined using standard LAUE and/or pole figure techniques employing x-ray diffraction. In this instance the x-ray beam is directed at an exposed surface of a crystal and a diffracted crystal lattice pattern is detected by interposing a detector on the diffracted beam path such that the LAUE and pole figure techniques can be employed. If access to the SX or DS crystal is not available for non-destructive testing thereof, for example such as if a surface of the SX or DS material is not exposed to the x-rays, the LAUE and pole figure techniques could not be used because the proper x-ray diffraction from the SX or DX material would be unable to be captured to determine the orientation of the material due to the interference of the x-rays by material masking the SX or DS material.
X-ray emitters and x-ray detectors and their use in x-ray diffraction techniques for measuring residual stresses in crystalline substances such as metal or ceramic materials are known. In these techniques, x-ray diffraction is utilized to subject the outer surface of the material to x-ray radiation with the resulting observed x-ray diffraction peak interpreted to arrive at a measurement of a strength related characteristic, i.e., stress, retained austenite, hardness of the part material, to show, for instance, the level of fatigue damage present in the material.
The current practices for the measurement of grain orientation in SX and DS materials where the SX or DS crystal obfuscated involve mechanical sectioning of the material to form a coupon which is then analyzed. Once the coupon is obtained, the crystalline structure of the material can then be analyzed using x-ray diffraction or visual metallographic inspection. However, since this process does require mechanical sectioning to perform the analysis, the actual material that will be present in, for example a turbine blade, is not directly tested. Rather, it is just a representative portion of the total material that was used to separately form the blade. Therefore, this method of fabricating a coupon for determining the crystalline structure would be considered a destructive method as the blade itself is destroyed and would be impossible to subsequently place the blade into service. Another method is to expose the underlying crystal by removing material preventing direct characterization of the SX or DS material via x-ray diffraction. However, removal of material to expose the underlying crystal could affect the strength characteristics of the component and for the purposes of this disclosure would also be considered a destructive method.